The present invention relates to a technique for measuring the element temperature of an air-fuel ratio sensor (including an oxygen sensor) equipped to an exhaust system of an internal combustion engine and utilized for controlling an air-fuel ratio of the engine, and to a technique for controlling a heater for heating a sensor element equipped to the air-fuel ratio sensor based on the measured element temperature.
Heretofore, there has been known an air-fuel ratio control device of an internal combustion engine for detecting an actual air-fuel ratio of the engine based on the oxygen concentration and the like in the exhaust using an air-fuel ratio sensor, and performing a feedback control of fuel supply quantity to the engine so that the actual air-fuel ratio reaches a target air-fuel ratio.
In order to perform the above-mentioned air-fuel ratio feedback control, it is required that the air-fuel ratio sensor is already activated. Since the air-fuel ratio sensor is activated when the element temperature reaches a predetermined activation temperature, the air-fuel ratio sensor is equipped with a heater for heating the sensor element, so as to control the element temperature to the target temperature by controlling the power supply to the heater.
Specifically, the internal resistance of the sensor element is measured, and based on the element temperature estimated from the measured resistance, a power supply amount to the heater is feedback controlled so that the element temperature reaches the target temperature (refer to Japanese Unexamined Patent Publication Nos. 8-278279, 61-122556,11-344466, etc.).
However, in a case where, for measuring the element temperature of the air-fuel ratio sensor (or the internal resistance of the sensor element related thereto), a predetermined voltage for measuring the element temperature (or for measuring the internal resistance) is applied to the sensor element, to measure the internal resistance based on the sensor output at that time, a voltage for detecting the air-fuel ratio is continuously output from the air-fuel ratio sensor even during internal resistance measurement (during application of measurement voltage). Therefore, if the sensor output is used as it is for measurement of internal resistance, an estimation error of the element temperature becomes too large.
Moreover, in recent years, the sensor element has been miniaturized and the capacity of the heater has been increased for the purposes of activating the sensor element quickly and maintaining the activated state securely, thereby improving the temperature follow-up capability of the sensor element against the power supply to the heater (relatively reducing the heat capacity of the sensor element). Therefore, especially in an air-fuel ratio control device wherein the power supply to the heater is controlled by performing a duty-control of the ON/OFF of the heater power supply, due to this ON/OFF of the heater power supply, the sensor element temperature is fluctuated momentarily and the internal resistance of the element is also fluctuated. This causes an error in the measurement of internal resistance, to lead to the deterioration of estimation accuracy of the element temperature.
Such deterioration of estimation accuracy of the element temperature not only causes the deterioration of the ability to control the element to the target temperature when controlling the power supply to the heater for heating the sensor element, but also causes the increase of the power consumption by the heater. It further causes a bad influence to the feedback control of the air-fuel ratio.
The present invention has been achieved in view of the above conventional problems and has an object to enable a more accurate measurement of the element temperature in an air-fuel ratio sensor.
Another object of the present invention is to enable an accurate judgment of the activation status of the air-fuel ratio sensor based on the accurately measured element temperature.
Yet another object of the present invention is to control a sensor element to the desired element temperature by controlling a heater for heating the sensor element based on the accurately measured element temperature.
Still another object of the present invention is to improve the accuracy of an air-fuel ratio feedback control based on the above heater control.
In order to achieve the above objects, the present invention is constituted as follows.
When applying a predetermined voltage for measuring the element temperature to the sensor element of the air-fuel ratio sensor, both a sensor output just before applied with the predetermined voltage as well as a sensor output being applied with the predetermined voltage are read in.
Based on both the sensor output just before applied with the predetermined voltage and the sensor output being applied with the voltage, the sensor element temperature is estimated.
With this constitution, since there can be considered an influence of voltage output for air-fuel ratio detection of the air-fuel ratio sensor by the sensor output just before applied with the voltage, the estimation accuracy of the sensor element temperature is improved, thereby enabling an accurate activation judgment and the like of the air-fuel ratio sensor.
Using the sensor output being applied with the predetermined voltage as a basis, and using the sensor output just before applied with the voltage as a correction parameter, the element temperature can be estimated, and further, by computing the internal resistance of the sensor element based on the sensor output, the element temperature can be estimated.
Even further, during internal resistance measurement of the sensor element of the air-fuel ratio sensor, a voltage applied to the heater for heating the sensor element is maintained to be constant.
Thereby, it becomes possible to prevent the fluctuation of sensor element temperature (in other words, the fluctuation of internal resistance of the sensor element), which is caused by the fluctuation of heater application voltage accompanying the control of the heater for heating the sensor element, so that the internal resistance of the sensor element can be measured accurately, and the element temperature can be accurately computed based on the measured internal resistance.
Further, it becomes possible to feedback control the heater for heating the sensor element, based on the accurately detected element temperature, so that the element temperature reaches a target temperature, thereby enabling the sensor element to be controlled to the desired temperature.